LEDs having a light-emitting layer composed of an aluminum gallium indium phosphide mixed crystal (compositional formula: AlXGaYInZP: 0≦X, Y, Z≦1, X+Y+Z=1) are known to emit light having a wavelength corresponding to green light to red light (see Y. Hosokawa et al., J. Crystal Growth, Vol. 221 (2000), Holland, p. 652-656).
As disclosed in aforementioned Y. Hosokawa et al., a light-emitting layer which emits visible light having a shorter wavelength is generally formed of AlXGaYInZP and exhibits a relatively large bandgap at room temperature of about 2 eV.
Generally, such a light-emitting layer has a heterojunction structure in which a cladding layer is joined to form a heterojunction for enhancing radiation recombination efficiency and attaining high-intensity light emission.
In a light-emitting layer formed of AlXGaYInZP, a cladding layer serving as a barrier layer which is joined to form a heterojunction is formed of AlXGaYInZP exhibiting a bandgap wider than that of the light-emitting layer. Although a carrier-confining effect is attained, such a cladding layer is generally insufficient for diffusing a current for operating a device (i.e., device operation current) over a wide range of the light-emitting layer, since the cladding layer is formed from a semiconductor layer exhibiting a wide bandgap.
To solve this problem, one prior art technique employs a current diffusion layer on a cladding layer for spreading device operational current over a wide range of the light-emitting layer (see U.S. Pat. No. 5,008,718).
The current diffusion layer is composed of a semiconductor material exhibiting a relatively narrow bandgap (e.g., smaller than the bandgap of the light-emitting layer) in order to widely diffuse device operation current.
For example, an orange-light-emitting or red-light-emitting compound semiconductor light-emitting diode is disclosed, in which a current diffusion layer composed of aluminum gallium arsenide (compositional formula: AlXGaYAs: 0≦X, Y≦1) is provided on the light-emitting layer composed of AlXGaYInZP (see, for example, page 4, paragraph [0010], in Japanese Patent Application Laid-Open (kokai) No. 11-4020).
However, the current diffusion layer formed of a semiconductor material exhibiting such a narrow bandgap by nature absorbs light emitted from the light-emitting layer. Therefore, even though the light-emitting layer has a heterojunction structure for attaining high-intensity light emission, high-intensity LEDs cannot be consistently and reliably produced so long as the LEDs have a conventional configuration in which a current diffusion layer is provided on the light-emitting area on the light extraction side.
Meanwhile, there has been disclosed an LED having a current diffusion layer formed of an optically transparent material such as indium tin complex oxide film (abbreviated as ITO) (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-144330).
Transparent oxide such as ITO can be employed as a material for forming a current diffusion layer serving also as a window layer through which light is extracted to the outside, by virtue of exhibiting wide bandgap and low resistance.
However, transparent oxide generally encounters difficulty in consistently maintaining Ohmic contact with a Group III-V compound semiconductor, and diffusion of device operation current over a wide area may fail to be attained.
To overcome the above problem, aforementioned Japanese Patent Application Laid-Open (kokai) No. 2001-144330 discloses a technique for diffusing device operation current over the light-emitting layer through employment of an electrode configuration in which a plurality of Ohmic electrodes are discretely provided on a cladding layer composed of a Group III-V compound semiconductor.
However, when the Ohmic electrodes are provided discretely, cumbersome steps are required for producing LEDs or other devices, which is problematic.
The current diffusion layer included in an LED is required to be formed of an optically transparent material so as to sufficiently diffuse device operation current over a wide area of the light-emitting layer and allow light emitted from the light-emitting layer to be extracted to the outside without absorbing the emitted light. In order to satisfy the above requirements, the current diffusion layer must exhibit a bandgap at room temperature which is wider than that of the light-emitting layer.
However, when the current diffusion layer is formed from a conventionally employed AlXGaYAs (0≦X, Y≦1), a conductor layer exhibiting sufficiently low resistance is difficult to form. In other words, there remains a drawback that a current diffusion layer suitably diffusing device operation current cannot be reliably formed.
Meanwhile, a Group II-VI compound semiconductor layer containing zinc (Zn) as a component element is susceptible to oxidation. In order to fabricate a light-emitting device of excellent operation reliability, such a Group II-VI semiconductor layer must be coated with an anti-oxidizing protective film. Such an additional operation would make the device production steps cumbersome.
An oxide material such as ITO, which is another material serving as a current diffusion layer, fails to reliably attain excellent Ohmic contact with a semiconductor such as a Group III-V compound semiconductor serving as a cladding layer. Thus, electric resistance between the cladding layer and the current diffusion layer formed of a transparent oxide material or a similar material increases, which may be disadvantageous in production of an LED exhibiting low forward voltage (Vf).